low voltage high current dc power supply

1 Introduction

In the electroplating industry, the general requirements of the power supply of the output voltage is low, and the current is very big. Power requirements are relatively high, the general is thousands of watts to tens of kilowatts. At present, such a large power of the electroplating power is generally used thyristor phase controlled rectifier. The disadvantages are the large size, low efficiency, high noise, low power factor, large output ripple, slow dynamic response, poor stability and so on.

In this paper, the switching power supply is introduced, and the output voltage from 0 to 12V and the current from 0 to 5000A can be adjusted continuously, and full load output power is 60kW. Due to the use of ZVT soft switch technology, and the use of a better cooling structure, the power of the indicators have met the requirements of the user, has now been put into production in small quantities.

2 main circuit topology

In view of such a high power output, high frequency inverter part adopts IGBT as power switching device of the full bridge topology, the main circuit as shown in Figure 1, including: power frequency three phase AC input, diode rectifier bridge, EMI filter, filter inductance capacitance, high frequency full bridge inverter, high frequency transformer, output rectifier, output LC filter, etc..

The DC capacitor Cb is used to balance the power of the transformer to the second value and prevent the bias. In consideration of the efficiency of the problem, the resonant inductor Ls only uses the leakage inductance of the transformer itself. Because if the inductor is too large, it will lead to high off voltage spike, which is very bad for the switch tube, but also increase the off loss. On the other hand, it can also cause serious duty cycle loss, which causes the current peak value of the switch device, which makes the performance of the system decrease.

3 zero voltage soft switch

High frequency full bridge inverter control mode for the phase shifted FB-ZVS control mode, control chip using Unitrode UC3875N. In the full load range, the zero voltage soft switching is achieved by the lead arm in the full load range. Figure 2. The driving voltage and the collector emitter voltage waveform of the lagging leg IGBT can be seen to achieve zero voltage turn-on.

Switching frequency selection 20kHz, this design can reduce the IGBT of the off loss, on the other hand, it can take into account the high frequency, so that the power transformer and output filter of the volume decreases.

4 capacitive power bus

In the first experiment, between the connection bus capacitor C5 and IGBT module for common power bus. In the experiment, the voltage and current of IGB on IGBT are all high frequency oscillations, and the primary voltage and current waveform of the transformer are collected in Figure 3. The reason is in parallel with the IGBT module on the surge absorbing parasitic capacitance and inductance power bus has high frequency resonance. One hour after the full load operation, power bus temperature is 38 DEG C, the capacitor C5 temperature is 24 DEG C.

5 a series of parallel transformers are used to realize the parallel structure of the output rectifier diodes.

In order to further reduce the loss, the output rectifier diodes using only current 400A, resistance to high voltage 80V Schottky diode in parallel. Moreover, the secondary output of each transformer uses a full wave rectification method. This sample is only one group of diodes that flow through the current. At the same time, in the secondary rectifier diode with on RC snubber network, to suppress by the transformer leakage and Schottky diode body capacitance caused by the parasitic oscillations. All these measures can reduce the power consumption and improve the efficiency.

For large current output, the general output of the rectifier diode in parallel. But because the Schottky diode is a negative temperature coefficient of the device, in parallel, the general should take into account the flow between them. There are many kinds of parallel modes of the diode, the graph a is the direct parallel mode, the B is a series of parallel connection mode, and the C is a series of dynamic current transformer. (the parallel connection of four diodes).

For the direct parallel method, the diode current is very poor, the output current is generally limited to tens of amps to hundreds of AMPS, not easy to do a thousand amps. In order to achieve the purpose of the current, the current can be used to achieve the purpose of the current flow. Because of the influence of proximity effect and skin effect, the average flow of the diode is changed with the output current, and the effect is poor. In order to achieve a good result, the resistance of the string is not too small, which brings great loss. For the series of dynamic current transformer, the parallel mode can achieve better results, but the production process of large current transformer is complex, the cost is high, and the leakage inductance and the lead inductance of the dynamic current transformer can be increased.

In order to overcome the shortcomings of the above parallel mode, the output rectifier diodes can achieve both automatic current sharing, reduce the loss, and can reduce the complexity of the production process, we have designed a novel high frequency power transformer, as shown in Figure 1. The transformer is composed of eight identical small transformer, the ratio was 4: 1, their primary series, while the secondary is the parallel structure. The transformer uses the primary cooling and secondary cooling method, which is considered to be different, and can greatly simplify the process of the transformer.

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